1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device.
2. Description of the Related Art
According to the background art, it has been well known that an ohmic contact (electric contact portion) between a silicon carbide semiconductor portion and a metal film (electrode) in a semiconductor device (silicon carbide semiconductor device) using a silicon carbide (SiC) semiconductor is formed by thermal treatment (annealing). A method for manufacturing the semiconductor device according to the background art will be described. FIG. 9 is a sectional view schematically showing a state in which the semiconductor device is being manufactured according to the background art. The state of a semiconductor substrate (semiconductor wafer) which is being annealed in a furnace is shown in FIG. 9.
As shown in FIG. 9, a device structure 102 is first formed on a front surface side of a semiconductor substrate made of silicon carbide (hereinafter referred to as “silicon carbide substrate”) 101. Then, a nickel (Ni) film 103 is formed on the back surface of the silicon carbide substrate. Next, the nickel film 103 is silicified by a rapid thermal annealing (RTA) furnace 104 for several minutes at a high temperature of about 1,000° C. or higher to form an ohmic contact with the silicon carbide substrate.
As a method for forming the ohmic contact with the silicon carbide substrate in this manner, the following method has been proposed. That is, a transition metal film is formed on a silicon carbide substrate, and the whole of the silicon carbide substrate is then heated by rapid thermal annealing for two minutes at a temperature of 1,000° C. to thereby form a silicide contact electrode having a high carbon content, e.g., see JP-A-2009-177102, paragraph [0017].
In addition, as another method, the following method has been proposed. That is, a nickel film is formed on a silicon substrate and thermal treatment is then performed thereon in a hydrogen (H2) gas atmosphere, e.g., see JP-A-2011-066060, paragraphs [0040] to [0042]. In JP-A-2011-066060, reaction between silicon atoms in the silicon substrate and nickel atoms in the nickel film is accelerated by the thermal treatment in the hydrogen gas atmosphere.
In addition, as a further method, the following method has been proposed. That is, a titanium (Ti) film, an aluminum (Al) film and a silicon film are formed sequentially on a silicon carbide substrate to thereby form a contact electrode, and titanium, aluminum and silicon contained in the contact electrode and silicon and carbon contained in the silicon carbide substrate are then alloyed by laser annealing, e.g., see JP-A-2012-099599, paragraphs [0042] to [0044].
In addition, as a further method, the following method has been proposed. That is, an oxide film (SiO2 film), a quantum dot made of silicon and a nickel thin film are formed sequentially on a silicon substrate, and a laminated film made up of the quantum dot and the nickel thin film is then formed into a nickel silicide dot by remote hydrogen plasma treatment, e.g., see WO 2009/118783, paragraphs [0056] to [0061].
However, according to the background-art manufacturing method, by use of the rapid thermal annealing furnace 104, silicification reaction between silicon atoms in the silicon carbide substrate 101 and nickel atoms in the nickel film 103 can proceed in a boundary between the silicon carbide substrate 101 and the nickel film 103. On this occasion, there is concern that excess carbon (C) atoms in the silicon carbide substrate 101 may be precipitated, clustered and left between the silicon carbide substrate 101 and the nickel film 103. The excess carbon atoms left between the silicon carbide substrate 101 and the nickel silicide film result in an increased contact resistance.
In addition, in the background-art manufacturing method, the whole of the silicon carbide substrate 101 (the whole of the device) is heated uniformly because it is not possible to selectively heat only a portion where the ohmic contact is formed, that is, the nickel film 103 or the boundary between the silicon carbide substrate 101 and the nickel film 103. Therefore, extra heat is applied to the vicinity of the boundary between the silicon carbide substrate 101 and an MOS gate (i.e., an insulating gate made of a metal, an oxide film and a semiconductor) formed as the device structure 102. There is a concern that, for example, device characteristic may deteriorate due to a thermal history caused by the heating.
The invention is directed to solving the foregoing concerns inherent in the background-art techniques. An object of the invention, therefore, is to provide a method for manufacturing a semiconductor device, in which an excellent ohmic contact can be formed and device characteristic can be prevented from deteriorating.